refactor(tpe): split multivariate sampler into focused submodules

- Extract engine.rs (core sampling logic) and trials.rs (trial processing)
- Deduplicate TPE sampling functions by delegating to tpe::common
- Gate persistence.rs imports with #[cfg(feature = "serde")]

src/study/persistence.rs

@@ -1,8 +1,12 @@
+#[cfg(feature = "serde")]
 use std::collections::HashMap;
 
+#[cfg(feature = "serde")]
 use crate::sampler::CompletedTrial;
+#[cfg(feature = "serde")]
 use crate::types::Direction;
 
+#[cfg(feature = "serde")]
 use super::Study;
 
 /// A serializable snapshot of a study's state.